Piezoelectric device



July 12, 1938 PIEZOELECTRIC DEVICE Filled June 26, 1935 A TTORNEY c. A.BIELENG 2,123,227

Patented in, 12, 1938 UNITED STATES PATENT OFFICE TelephoneLaboratories,

Incorporated, New

York, N. Y., a corporation of New York Application June 26, 1935, SerialNo. 28,414

Claims. (Cl. 171-427) This invention relates to piezoelectric elementsand particularly to metallic coatings for such elements.

An object of the invention is to improve the per- 5 formance ofpiezoelectric crystals.

Another object of the invention is to facilitate the conductiveassociation of piezoelectric crystals with electrical circuits.

A more specific object of the invention is to provide a metallic coatingon the surface of a piezoelectric crystal.

Piezoelectric crystals are, as is well known, commonlyused as elementsin electrical circuits where a constant frequency is essential and areof 5 increasing importance inconnection withwave filters and similarelectrical networks. when the piezoelectric elements are used in suchelectrical circuits, it is necessary, of course, that some means beprovided whereby the piezoelectric ele- 2g ment may be operativelyassociated with the circuit. It is common practice to provide for thisPurpose a metallic coating on one or more of the surfaces of thepiezoelectric crystal element, this coating being electrically connectedto the crystal 25 surface to which it is applied. The desired conductorsof the circuit may then be attached, for

example by soldering, to the coatings. These coatings are commonlyreferred to as the electrodes of the crystal.

30 Certain somewhat stringent requirements exist with.respect tocoatings of the nature referred to above. For example, for some purposesin order that the coating may perform its role of an electrodesatisfactorily, it must extend evenly over 35 the surface of the crystaland must be in intimate contact therewith.- Moreover, the process ofapplying the coating must be such that the structure of the crystal isnot damaged. The coating, in addition to its use as an electrode may be40 utilized to regulate certain characteristics of performance of thecrystal, as outlined for example in Hulbert United States Patent1,848,630 and the process of applying the coating should therefore be ofsuch a nature that the thickness 45 of the coating be uniform and easilyregulated within narrow limits.

According to a feature of the invention the metalliccoating is appliedto the surface of a piezoelectric crystal element by the spraying 50process commonly referred to as the Schoop process. It has been foundthat a coating applied by this process meets the requirements referredto above in an economical and otherwise satisfactory manner particularlywith respect to Rochelle salt 55 crystals although the process may alsobe used to the coating and the crystal.

advantage in coating other types ofpiezoelect'ric crystal elements.Methods previously followed in i the application of electrode coatingsto piezoelectric crystal elements have proven generally unsatisfactorywhen used in connection with 5,

Rochelle salt crystals. In some instances where the known methodinvolved considerable heat, dehydration of the crystal resulted while,in instances of methods involving less heat, it was found diflicult toobtain a firm adherence between 10 However with respect to the Schoopprocess,'the Rochelle salt body is not damaged in any way thereby, theresulting coating adheres satisfactorily and the electricalcharacteristics of the piezoelectric device produced thereby are suchthat the device is particularly adaptable for use in electrical filtersand similar circuits. The value of r, for-example, is comparatively ;lowwhile the value of "Q is substantially higher than. that obtained withRochelle salt piezoelectric elements, the electrodes of which have beenapplied by other known methods. (The terms 1'" and Q are derived fromconsideration of the so-called equivalent electrical circuit of thepiezoelectric device which circuit is considered to comprise aninductance in series with a capacity C1 and shunted by a capacity Co, 1'being the ratio of Co to C1 and Q being the ratio of the reactance tothe resonance resistance of the equivalent circuit referred to. Thetheory and derivation of the equivalent electrical circuit ofpiezoelectric devices is explained on page 45 of Quartz Resonators andOscillators by P. Vigoureux published by His Majestys Stationery Oflice,London.)

In accordance with another feature of the inventionrefinemen'ts areintroduced in the design of the pistol used whereby it is better adaptedto the application of the coating in the accurate and easily regulatedmanner required. 40 In accordance with an additional featureof theinvention a novel conveying arrangement is V utilized for feeding thecrystal plates to the spraying device whereby the thickness of thecoating may be automatically controlled within certain narrow limits.

A thorough understanding of the invention and of the variousadvantageous features thereof may be gained from consideration of thefollowing detailed description and the annexed drawing in which:

Fig. 1 is 'a perspective view of a piezoelectric crystal plate, thesurfaceof which has been coated by the process contemplated by thepresent invention; I

Fig. 2 is an end view'of the crystal shown in Fig.

Fig. 3 is a side elevation of .a Schoops pistol of the. type adapted'for the coating process contem- -plated;'

-rangement adapted to be used in the coating process contemplated by theinvention.

Referring now to the drawing, a piezoelectric plate is shown in Fig. 1which may be, for example, of the Rochelle salt type. A metallicelectrode coating i2 is shown applied to the upper major surface of theplate, a portion H of the plate being left uncoated, thereby effectivelyseparating the coating I2 into two distinct'sections.

If desired a coating of similar or difierent pattern maybe applied tothe lower major surface of the plate as Well as to the ends and sidesthereof.

As shown most clearly in Fig.2, certain of the edges'of the plate havebeen rounded out, it having been found that this procedure is effectivein preventing fracturing of the coating and chipping of the crystalitself during the preparation and use of the plate.

In accordance with the invention the coating I2 is preferably applied tothe plate by'means of a Schoops pistol of the general type shown in Fig.3. This pistol comprises a casing l5 which encloses a feeding mechanism(not shown) to which wire I6 is supplied. A pistol-grip handle ll,control trigger I8 and supply pipes 2i and 22 are also provided.

In accordance with the usual method pf operation of the pistol, the wireI6 is fed through an opening in nozzle 23 (Fig. 4) and is reduced to aliquid state by an oxy-hydrogen flame formed by the combustion of thesegases in chamber 24. The metal particles are then projected throughorifice 28 of cap 25 at a high velocity, air, supplied under pressurethrough chamber 26, forcing the particles out through the orifice.

According to a feature of the invention orifice 28 provided in cap 25 isnot round but is, as shown -most clearly in Fig. 5, elongated orflattened. It

has been found that the provision of an orifice of this particular shapegreatly enhances the value of the pistol for fine work such asthat'involved in crystal coating as it permits the application of theparticles in a thin well defined patternmuch as if a small paint brushwere used. Coatings may therefore be applied in separate distinctsections as shown in Fig. 1, whereas the use of circular orifices of thetype previously provided on Schoops pistols does not permit this finedefinition of pattern.

previously used, it has been observed that the tips became damaged inuse by the flame and soon presented a distorted orifice. While this didnot prove particularly troublesome in coarse work, it

work such as crystal coating as it is paramount,

nozzle and of the cap remain the same.

It has been observed further that, with respect to the type ofv pistolspreviously used, the "uni formity of the coating was destroyed duetothenozzle partially clogging particularly at low velocities. investigationthat this trouble can be overcome by providing a particularly smoothsurface, such as obtained, for example, by chromium plating, on theinside surface of cap 25.

Fig. '6, which shows a piezoelectric plate in the process of beingcoated, well illustrates the adaptability of a pistol provided with therefinements referred to above to fine 'work such as crystal coating. Asshown the particles are projected in a definite well defined patternthereby permitting the production of separate section coatings of thenatureshown in Fig. l. The thickness of the coating produced, moreover,is uniform and easily regulated.

Referring now to Fig. 7 there is shown, schematically, a conveyingarrangement adaptable for use in the crystal spraying process referredto above. The arrangement contemplates theprovision of a conveyor belt38 which is supported on Applicant has found, after considerable ifuniform particle size and definite coating thickness are to be attained,that the orificesofthe.

39, polarized relay 52, meter d5, batteries 36 and ll and a power source58 are also included.

- It will be noticed that one or more of the three fixed resistances 35,36'or 31 is included in the operating circuit of driving motor 38, theparticular resistance or resistances included depending upon theposition of armature 5! of relay 32.

When armature 5| is in the neutral position of normal speed. When thehigher resistance 31 is included, however, the speed of the motor willdecrease and when resistances 36 and 36 are included in parallel theresulting lower efiective resistance in the motor circuit will result inan increase-in the operating speed of the ,motor.

The left-hand winding of relay 42 is energized by current from battery41, the strength'of this energizing current being varied as desired byoperation of rheostat 38 while the right-hand winding of the relay is,energized by current from battery 46, the strength of this current beingvariedv by operation of rheostat 39. The last mentioned energizingcircuit includes contact members 54 and 55, each of which is provided,with a roller which is adapted to be placed in engagement with a coatedpiezoelectric plate.

The coating of the plate may in this way be associated with theenergizing circuit for the righthand winding of relay 42. Thearrangement of the relay circuit is such that, if the resistance of acoating be within certain predetermined limits thereby indicatingthecorrectcoating thickness, the current flowing in the energizingcircuit will be such that armature 5| willnot be moved from its normal'p osition shown in Fig.- 1. Should, however, the resistance of thecoating be less than the predetermined minimum, thereby indicating acoating of too great a thickness, the increase of current flowingthrough the right-hand winding of relay 42 will cause armature 5| toswing to the "up position in which position, as pointed out above,resistances 35 and 36 in parallel are connected in the motor circuit andthe speed of motor 34 is increased. Conversely, should the resistance ofthe coating exceed the predetermined maximum limit thereby indicating acoating of insufllcient thickness, armature 5| is moved to the downposition and the higherresistance 31 is brought into the motor circuit.

In order to illustrate the operation of the arrangement of Fig. v'I, letus assume that piezoelectric plate 56 has been coated and that plate 51is in position under the spray emanating from nozzle of the Schoopspistol; In order to simplify the description, it is assumed that thenozzle is so adjusted in this instance that the path of the particlessent forth is sufliciently extended to cover the entire plate. Parts ofthe plate to be left free of the coating may in such an instance beprotected by a mat.

The energizing circuits for the right-hand and left-hand windings ofrelay 42 havingbeen properly adjusted by manipulation of respectiverheostats 33 and 33, members 34 and 53 are placed in contact with thecoating of plate 53 and driving motor34 is set into operation therebycausing" rotation of drum 33 and movement of belt 3| and plate 51 underthe spray of metal particles projected by the gun. Assuming, first, thatthe thickness of the coating on plate 53 is correct within thepredetermined limits, the resistance of the energizing circuit ofright-hand winding of relay 42 remains such that armature Si is notmoved from its normal position, the resistance of the motor circuit isnot changed and the motor continues at the predetermined normal" speed.

Assuming, however, that the thickness of the coating on plate 56 is toogreat, obviously the speed at which the belt is moving shouldbeincreased so thatplate' 51 and the plates following it will be movedthrough the spray at an increased speed in order that the thickness ofthe coating applied to these plates may be "out do This desiredincrease-in speed is automatically attained as the decreased resistancein the energizing circuit of the right-hand winding of relay 42 duetothe undue thickness of the coating results in moving armature 5| tothe "up position, in which position, as previously pointed out,resistances 35 and 36 in parallel are connected into the motor circuit.The resulting drop in the resistance of the motor circuit results in anincrease in speed of motor 34 and 8, corresponding increase in the speedof the conveying belt.

Had the coating thickness been found insufliclent the proper remedywould, of course, have been to decrease the speed of the belt in orderthat the subsequent plates might receive a coating of proper thickness.In such an instance the increase of resistance in the energizing circuitof the right-hand winding of relay 42 would be effective to swingarmature 5! to the down position, thereby connecting resistance 31 inthe motor circuit. The magnitude of resistance 31 is higher than that ofresistance 36 and the increase of resistance in'the motor circuit would,of course, cause a decrease in the operating speed of motor 34.

Insulating strips 8| are placed on the belt between each two adjacentpiezoelectric plates.

Contact members 54 and 55 engage these strips after leaving one plateand before engaging the subsequent plate. While a momentary interruptionof the energizing circuit of the right-hand winding of relay 42 resultsfrom engagement of the contact members with these insulating strips,relay 42 is slow-to-operate and the armature does not change itsposition during the comparatively short time the circuit is interrupted.

While certain specific embodiments of the invention have been selectedfor detailed description, the invention is not, of course, limited inits application to the specific embodiments described. These embodimentsshould be considered illustrative of the invention rather thanrestrictive thereof to prevent fracturing of said coating, the

coating on at least one of said surfaces being separated into aplurality of sections to form a plurality of electrodes.

4. A piezoelectric crystal plate having metallic coating formed integraltherewith, an edge of said plate between a major and another surfacethereof being rounded to prevent fracturing of said coating.

5. A piezoelectric crystal plate having metallic coating formed integraltherewith, an edge of said plate between a major and another surfacethereof being rounded to prevent fracturing of said coating, the coatingon said major surface being separated into a plurality of sections toform a plurality of electrodes.

CARL A. BIELING.

